Known on Russian .308 Win and .30-06 steel cased ammunition. The official answer is that it is to prevent the steel cases from sticking in the chamber. A technical explanation of how the groove accomplishes this is still unknown. I've seen fired cases where the groove has been completely blown out and no longer visible to completely un-changed. We fired some of this in an M-1 rifle and found it to be incompatible. Very violent extraction and a huge fireball. Not loaded with a powder compatible with the M-1.